Fuse production method, fuse, circuit board production method and circuit board

ABSTRACT

A fuse production method includes the steps of forming a liquid film of a dispersion liquid, in which metal nanoparticles are dispersed in a solvent, on a principal surface of a substrate containing at least an organic substance, heating the liquid film so as to vaporize the solvent to melt or sinter the metal nanoparticles and to soften or melt the principal surface, and forming a fuse film on the principal surface by fusing the melted or sintered metal nanoparticles and the softened or melted principal surface with each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of InternationalApplication number PCT/JP2015/060874, filed on Apr. 7, 2015. The contentof this application is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a fuse production method, a fuse, acircuit board production method and a circuit board.

Fuses are used to prevent the occurrence of circuit breakdown due to aninflow of excess current caused by a failure, or the like, in anelectronic device. Specifically, a fuse has a fuse film bonded to asubstrate, and when an abnormal current flows in a circuit, the fusefilm is blown to cut off the circuit to prevent the circuit from beingbroken.

When a bonding between the substrate and the fuse film is weak, there isa possibility that the fuse film is peeled from the substrate and breakseven when no abnormal current flows. In particular, since conductionoccurs repeatedly through the fuse film, expansion and contraction ofthe fuse film frequently occurs between the fuse film and the substratebecause the fuse film and the substrate have a different expansioncoefficient with respect to the rise in temperature, and so the fusefilm is easily peeled from the substrate. Therefore, there is a demandto firmly bond the fuse film to the substrate in order to prevent thefuse film from being peeled off.

Patent Document 1 (Japanese Patent Application Publication No.2008-200875) proposes, in order to enhance an adhesion between aninsulating substrate and a metal thin film in a circuit board, a methodof causing a metal oxide to exist at a contact interface between aninsulating resin layer, which exists between the insulating substrateand the metal thin film, and the metal thin film.

As electronic devices incorporating fuses are becoming smaller andlighter, further downsizing and weight reduction of the fuses arerequired. However, when the technique of Patent Document 1 is applied toa small fuse, it is necessary to make the surface roughness of thecontact interface between the insulating resin layer and the metal thinfilm be 100 (nm) or less, which incurs a great cost.

Further, the production method disclosed in Patent Document 1 includesperforming a heat treatment in an atmosphere containing an oxidizingagent. On this occasion, it is necessary to adjust the oxygenconcentration to about 20 to 2000 (ppm), which requires expensiveequipment.

BRIEF SUMMARY OF THE INVENTION

This invention focuses on these points, and the invention produces afuse whose fuse film is resistant to being peeled from a substrate at alow cost.

One aspect of the present invention provides a fuse production methodcomprising the steps of forming a liquid film of a dispersion liquid, inwhich metal nanoparticles are dispersed in a solvent, on a principalsurface of a substrate containing at least an organic substance, heatingthe liquid film so as to vaporize the solvent to melt or sinter themetal nanoparticles and to soften or melt the principal surface, andforming a fuse film on the principal surface by fusing the melted orsintered metal nanoparticles and the softened or melted principalsurface with each other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a fuse 900 according tocomparative example 1.

FIG. 2 is a schematic planar view of the fuse 900.

FIG. 3 is a graph showing a pulse life test result of the fuse 900according to comparative example 1.

FIG. 4 is a graph showing a pre-arcing time-current characteristic curveof the fuse 900 according to comparative example 1.

FIG. 5 is a diagram showing a pulse waveform inputted during a pulselife test.

FIG. 6 is a graph showing the transition of the resistance value when apulse life test was performed.

FIG. 7 is a schematic sectional view of a fuse 1 according to oneexemplary embodiment of the present invention.

FIG. 8 is a schematic planar view of the fuse 1.

FIG. 9 is a captured image showing a bonding state between the fuse film20 and the support substrate 10.

FIG. 10 is a captured image showing a bonding state between the fusefilm 20 and the support substrate 10.

FIG. 11 is a graph showing pulse life test results of the fuse 1according to the present exemplary embodiment and fuses according tocomparative examples 2 and 3.

FIG. 12 is a graph showing pulse life test results of the fuse 1according to the present exemplary embodiment and the fuse 900 accordingto comparative example 1.

FIG. 13 is a flowchart showing a production process of the fuse 1.

FIG. 14 is a schematic view showing an ink film 110 formed on acomposite substrate 100.

FIG. 15 is a schematic diagram showing an example of a configuration ofa laser irradiation apparatus 200.

FIG. 16 is a flowchart showing details of a fuse film/internal terminalforming process.

FIG. 17 is a diagram showing the composite substrate 100 after formationof a fuse film/internal terminal.

FIG. 18 is a diagram showing a formation state of a fuse film 120 andinternal terminal groups 130.

FIG. 19 is a graph showing the relationship between the thickness t(i)of the ink film 110 before laser irradiation and the thickness t of thefuse film 120 after irradiation.

FIG. 20 is a graph showing the relationship between a spot diameter φ ofthe laser light and the width w of the fuse film 120.

FIG. 21 is a flowchart showing the details of the post process.

FIG. 22 is a diagram showing a formation state of an overcoat 140 on asub-assembly 118.

FIG. 23 is a diagram showing a formation state of external terminals 151and 152.

FIG. 24 is a diagram for describing the stamping of a seal on theovercoat 140.

FIG. 25 is a schematic cross-sectional view of a circuit board 500according to one exemplary embodiment of the present invention.

FIG. 26 is a schematic planar view of the circuit board 500.

FIG. 27 is a flowchart showing a production process of the circuit board500.

DETAILED DESCRIPTION OF THE INVENTION

In the following, the description will be given in the order indicatedbelow.

1. Comparative Example

-   -   1-1. Configuration of fuse according to comparative example    -   1-2. Pulse life test    -   1-3. Heat cycle test    -   1-4. Peeling of fuse film    -   1-5. Peeling of internal terminal

2. Configuration of fuse

3. Fuse production method

4. Variation

5. Configuration of circuit board

6. Circuit board production method

1. Comparative Example

A fuse according to a comparative example is described before thedescription of a fuse according to the present invention. Hereinafter,the configuration of the fuse according to the comparative example isdescribed, and then problems that occur in the fuse according to thecomparative example are described.

(1-1. Configuration of Fuse According to Comparative Example)

FIG. 1 is a schematic cross-sectional view of a fuse 900 according tocomparative example 1. FIG. 2 is a schematic planar view of the fuse900. As shown in FIG. 1 and FIG. 2, the fuse 900 according tocomparative example 1 includes a support substrate 910, a fuse film 920,internal terminals 930, an overcoat 940, and external terminals 950.

The fuse 900 is inserted into a circuit in series and, for example, whenan excess current flows into the circuit, the fuse film 920 is blown toprotect the circuit. The length L1 of the fuse 900 in the longitudinaldirection is about 1.6 (mm), the length L2 of the fuse 900 in the widthdirection is about 0.8 (mm), and the thickness L3 of the fuse 900 isabout 0.7 (mm). Further, the weight of the fuse 900 is about 1.7 (mg).

The support substrate 910 supports the fuse film 920 and the internalterminals 930. The support substrate 910 is a substrate consisting of anorganic compound such as an epoxy substrate containing fiberglass. Thefuse film 920 is formed on a principal surface 912 of the supportsubstrate 910. The fuse film 920 is a conductor and the fuse film 920herein is made of silver. Each end of the fuse film 920 in thelongitudinal direction is electrically connected to an internal terminal930.

The internal terminals 930 are conductors and are formed at each end ofthe fuse film 920 in the longitudinal direction on the principal surface912 of the support substrate 910. The overcoat 940 coats the fuse film920 and a portion of the internal terminals 930. The overcoat 940 ismade of, for example, epoxy resin. The external terminals 950 are madeof, for example, silver and are formed on the internal terminals 930 soas to be connected to the internal terminals 930.

In the fuse 900, a rush current (also referred to as an inrush current)may occur at the time of switching on and off the power supply to thecircuit. The rush current may occur due to, for example, charging anddischarging of a capacitor inserted in the circuit. The rush currentgenerally has a spike-shaped current waveform, and has a characteristicthat the current peak is high and the conduction time is short. Also,the fuse 900, which should not be blown, is sometimes blown in somecases due to the rush current.

For this reason, the fuse 900 is required to be blown when an abnormalcurrent flows, but it is required to resist the rush current and not tobe blown by the rush current. As a test method of the durability of thefuse 900 against the rush current, a pulse life test that inputs apredetermined pulse waveform to the fuse 900 is used. Durability againstthe rush current can be evaluated by obtaining the pulse life of fuse900 with the pulse life test.

(1-2. Pulse Life Test)

Here, the pulse life of the fuse 900 according to comparison example 1is described with reference to FIG. 3.

FIG. 3 is a graph showing a pulse life test result of the fuse 900according to comparative example 1. The horizontal axis of the graphshows the current load ratio η (%) and the vertical axis shows the pulselife M (times). The pulse life M is a number of a pulse waveform thatcan be inputted to the fuse film 920 before the fuse film 920 is blown.

The current load ratio η is set as below. FIG. 4 is a graph showing apre-arcing time-current characteristic curve of the fuse 900 accordingto comparative example 1. The horizontal axis of the graph shows theconduction time T and the vertical line shows the conduction current I.As can be seen from FIG. 4, as the conduction time T increases, thepre-arcing time-current characteristic of the fuse 900 shows a tendencyfor the conduction current I to decrease.

FIG. 5 is a diagram showing the pulse waveform inputted during the pulselife test. The conduction time of the pulse waveform is T_(p) and theconduction current of the pulse waveform is I_(p). When the conductiontime T_(p) of the pulse waveform is set on the horizontal axis of thegraph of FIG. 4, the fuse 900 is blown at a point P where the conductioncurrent I is I₁. In such a case, the current load ratio η of the fuse900 is I_(p)/I₁. Therefore, the current load ratio 11 is proportional tothe magnitude of the conduction current I_(p) of the pulse waveform.

As can be seen from the graph shown in FIG. 3, the pulse life of thefuse 900 according to comparative example 1 decreases as the currentload ratio η increases. For example, when the current load ratio η is100(%), the fuse film 920 is blown when the pulse waveform is inputtedonce, and when the current load ratio η is 90(%), the fuse film 920 isblown when the number of the inputted pulse waveform reaches about 20times. Since the pulse waveform is repeatedly inputted to the fuse 900normally, improvement of the pulse life of the fuse 900 is required.

(1-3. Heat Cycle Test)

The fuse 900 according to comparative example 1 receives a temperaturechange repeatedly over a long period depending on the surroundingenvironment and conditions of use. As a reliability test of such a fuse900, a known heat cycle test is performed. By performing the heat cycletest, it is possible to evaluate, for example, damage, abnormalresistance change, and the like of the internal terminals 930 and theexternal terminals 950 of the fuse 900.

Here, as a heat cycle test, a temperature change from −40° C. to 125° C.was repeated twenty times for the fuse 900. As a result of the test, theresistance value between the external terminals 950 of the fuse 900 roseto about twice or more of the resistance value before the test.Accordingly, the reliability of the fuse 900 is insufficient, and animprovement is required.

(1-4. Peeling of Fuse Film)

There are cases where the fuse film 920 of the fuse 900 according tocomparative example 1 is peeled from the support substrate 910 byrepeating conducting states and non-conducting states, and the fuse 900is blown. Hereinafter, a mechanism by which the fuse film 920 is peeledfrom the support substrate 910 is described.

The fuse film 920 generates heat when conducting. Further, the supportsubstrate 910 that is bonded to the fuse film 920 receives the heat fromthe heated fuse film 920. As a result, the fuse film 920 and the supportsubstrate 910 are thermally expanded due to the increase of temperatureby Δθ at a bonding interface between the support substrate 910 and thefuse film 920.

Here, when the linear expansion coefficient of the fuse film 920 is α₁and the linear expansion coefficient of the support substrate 910 is α₂,a misalignment force F of the following expression (1) is generated atthe bonding interface.F=K ₁ ·K ₂·(α₁−α₂)·Δθ  (1)

It should be noted that K₁ is a constant determined from the shape andsize of the fuse film 920 and the support substrate 910, and K₂ is aconstant determined from the material and physical properties of thefuse film 920 and the support substrate 910.

Normally, the linear expansion coefficient α₂ of the support substrate910 made of an organic compound is larger than the linear expansioncoefficient α₁ of the fuse film 920 made of metal. Therefore, when thetemperature of the fuse film 920 and the support substrate 910 isincreased due to conduction, the misalignment force F generated at thebonding interface acts as a tensile force on the fuse film 920 and actsas a compressive force on the support substrate 910.

On the other hand, when the conduction is stopped, the temperature ofthe fuse film 920 and the support substrate 910 returns to the originaltemperature due to heat dissipation, and the misalignment force Fgenerated at the bonding interface disappears. Therefore, when theconducting states and non-conducting states are repeated, generation anddisappearance of the misalignment force F at the bonding interface arerepeated, and as a result, the fuse film 920 is peeled from the supportsubstrate 910.

The inventors of the present invention conducted a pulse life test toconfirm a phenomenon of peeling of the fuse film 920. Hereinafter, thetest result is described, and the details of the peeling phenomenon ofthe fuse film 920 are described.

FIG. 6 is a graph showing the transition of the resistance value whenthe pulse life test was performed. The horizontal axis of the graphshows the number N of pulses inputted to the fuse 900, and the verticalaxis shows the cold resistance ratio K of the fuse 900. Further, thehorizontal axis has a logarithmic scale. The cold resistance ratio K isexpressed by the following expression (2).K=R/R ₀  (2)

Here, R₀ indicates the initial cold resistance of fuse 900, that is, theresistance when N=0, and R indicates the cold resistance when apredetermined pulse is inputted N times to the fuse 900. The coldresistance is the resistance value of the fuse 900 measured at roomtemperature in a state where conduction is stopped.

As can be seen from FIG. 6, the cold resistance ratio K decreases in thesection where the input number N is from N₁ to N₂, and the coldresistance ratio K increases in the section where the input number N isfrom N₂ to N₄. On this occasion, the cold resistance ratio K was 1 atN=N₃ (D point), and the fuse film 920 was blown when N=N₄ (E point).

The following points were comprehended with the pulse life test.Specifically, when the cold resistance of the fuse 900 is increased, theheat generated by the fuse film 920 increases. Then, the temperature ofthe fuse film 920 increases, and the cold resistance further increases.The progress of this series of processes is accelerated as the number Nof inputted pulses increases. In the pulse life test, the followingpoints were confirmed by observing the fuse film 920. Specifically, itwas found that, in the section where the number N of inputted pulses wasfrom N₃ to N₄, the fuse film 920 was peeled from the bonding surfacewith the support substrate 910 and melted at the peeled portion.

When the temperature of the fuse film 920 rises, the misalignment forceF increases because Δθ of the above-described expression (1) increases.Then, the misalignment force F becomes larger than the bonding strengthbetween the fuse film 920 and the support substrate 910, and so the fusefilm 920 is displaced from the support substrate 910 and is peeled fromthe support substrate 910. It should be noted that it was found that thebonding strength of the fuse film 920 with the support substrate 910decreased as a result of repeating conducting states and non-conductingstates, compared with the bonding strength at the time before theconduction was started. Therefore, an increase in the misalignment forceF and a decrease in the bonding strength occur by repeating conductingstates and non-conducting states, and the fuse film 920 is easily peeledfrom the support substrate 910.

It was found that the temperature of the fuse film 920 peeled from thesupport substrate 910 due to the conduction was remarkably higher thanthe temperature before the peeling. It should be noted that the reasonfor the large increase of temperature of the fuse film 920 after thepeeling is that the transference of the heat, which is generated in thefuse film 920 due to the conduction, from the fuse film 920 to thesupport substrate 910 decreases when the fuse film 920 is peeled fromthe support substrate 910. The peeled fuse film 920 becomes easy to beblown since the heat generation and resistance also become easy toincrease after that.

(1-5. Peeling of an Internal Terminal)

As described above, when a heat cycle test is conducted, the resistancevalue of the fuse 900 is greatly increased. As a result of investigatingthe fuse 900 after the test, it was found that the internal terminals930 were peeled from the support substrate 910 at the time of the heatcycle test, which was a cause of an increase in the resistance value.

Specifically, at the time of the heat cycle test, a misalignment force Fis generated at the respective bonding interfaces between the supportsubstrate 910, the fuse film 920, and the internal terminals 930, whichhave different linear expansion coefficients, and the fuse film 920 andthe internal terminals 930 are peeled from the support substrate 910.Then, when the internal terminals 930 are peeled from the supportsubstrate 910, a misalignment force F is also generated at the bondinginterface between the internal terminals 930 and the external terminals950, and so the electrical resistance at the misaligned bondinginterfaces rises, and as a result, the resistance value of the fuse 900also increases. Therefore, in order to increase the reliability duringthe heat cycle test of the fuse 900, it is preferable to firmly bond theinternal terminals 930 to the support substrate 910.

2. Configuration of Fuse

The configuration of the fuse 1 according to one exemplary embodiment ofthe present invention is described with reference to FIG. 7 and FIG. 8.FIG. 7 is a schematic sectional view of the fuse 1 according to oneexemplary embodiment of the present invention. FIG. 8 is a schematicplanar view of the fuse 1.

The fuse 1 is surface-mounted on a circuit board or the like of anelectronic device, and is blown when abnormal current flows in thecircuit. As shown in FIGS. 7 and 8, the fuse 1 includes a supportsubstrate 10, a fuse film 20, internal terminal groups 31 and 32, anovercoat 40, and external terminals 51 and 52. The fuse 1 iselectrically connected to the circuit board via the external terminals51 and 52, and current is supplied from the circuit board to the fusefilm 20 via the external terminals 51 and 52.

The length L1 of the fuse 1 in the longitudinal direction is about 1.6(mm), and the length L2 in the width direction is about 0.8 (mm), andthe thickness L3 is about 0.4 (mm). The lengths L1 and L2 are the sameas those of the fuse 900 according to comparative example 1 shown inFIG. 2, but the thickness L3 is smaller than the thickness of the fuse900. The weight of the fuse 1 is about 0.9 (mg), which is smaller thanthe weight of the fuse 900. As described above, the fuse 1 is a fusewhose thickness and weight are reduced.

The support substrate 10 is a substrate that supports the fuse film 20and the internal terminal groups 31 and 32. The support substrate 10 isa substrate made of, for example, an organic compound, and the supportsubstrate 10 here is a non-thermoplastic polyimide resin substrate. Thethickness of the support substrate 10 is about 250 (μm).

The fuse film 20 is bonded to a principal surface 12 of the supportsubstrate 10. The fuse film 20, which will be described later in detail,is formed on the principal surface 12 by irradiating the ink filmcontaining metal nanoparticles with laser light. As the metalnanoparticles, for example, silver nanoparticles are used. The width wof the fuse film 20 is about 10 (μm), and the thickness t of the fusefilm 20 is about 0.25 (μm).

The fuse film 20 has entering parts 22 (FIG. 9) that enter inside of thesupport substrate 10 from the bonding surface with the principal surface12. The entering parts 22 are formed by fusing the metal nanoparticles,which are molten or sintered by being irradiated with the laser light atthe time of forming the fuse film 20, with the principal surface 12 ofthe support substrate 10.

FIG. 9 is a captured image showing the bonding state between the fusefilm 20 and the support substrate 10. The captured image shown in FIG. 9is an image observed with known scanning electron microscopy (SEM). TheSEM observation in the present exemplary embodiment was carried outusing ULTRA55, which is an SEM manufactured by Carl Zeiss Corporation,and NSS312E (EDX), which is an energy dispersive X-ray spectrometermanufactured by Thermo Fisher Company.

In FIG. 9, the white portion is the fuse film 20 and the black portionunder the white portion is the support substrate 10. As can be seen fromFIG. 9, it can be confirmed that a plurality of entering parts 22entering into the support substrate 10 from the fuse film 20 isdispersed. The shapes and sizes of the plurality of entering parts 22are slightly different from each other.

The entering parts 22 are engaged with the inside of the supportsubstrate 10. For example, the entering part 22 has a hook shape, andthe hook-shaped portion is engaged with the inside of the supportsubstrate 10. The shape of the entering part 22 is not limited to thehook shape, and the shape may be, for example, a spherical shape.Further, the width of the distal end side of the entering part 22 islarger than the width of the entering part 22 at the root part side. Inthis manner, it is easy to maintain the state where the entering parts22 are engaged with the support substrate 10, and so the fuse film 20 isfirmly bonded to the support substrate 10. As a result, it becomesdifficult for the fuse film 20 to be deviated from the principal surface12 of the support substrate 10, and the fuse film 20 is less likely tobe peeled from the principal surface 12 of the support substrate 10.

The support substrate 10 also has entering parts 14 (FIG. 10) that enterthe inside of the fuse film 20 from the principal surface 12. Aplurality of entering parts 14 is dispersedly formed and is engaged withthe inside of the fuse film 20. Since the entering parts 14 are formedin addition to the entering parts 22 in such a way, it is possible tofurther strengthen the bonding between the fuse film 20 and the supportsubstrate 10.

FIG. 10 is a captured image showing a bonding state between the fusefilm 20 and the support substrate 10. FIG. 10 is also an image observedby SEM similarly to FIG. 9. It should be noted that the captured imageshown in FIG. 10 shows a bonding state at the end portion in thelongitudinal direction of the fuse film 920 unlike FIG. 9 that shows thebonding state on the position near the center in the longitudinaldirection of the fuse film 920. As can be seen from FIG. 10, at the endportion in the longitudinal direction of the fuse film 920, the enteringparts 14 that enter the inside of the fuse film 20 from the supportsubstrate 10 in addition to the entering parts 22 are seen.

It should be noted that, in the fuse 900 according to comparativeexample 1 described above, an entering part is not formed in the fusefilm 920 and the support substrate 910, and the bonding surface of thefuse film 920 and the principal surface of the support substrate 910 issmooth. Therefore, in the fuse 900, the fuse film 920 is easily peeledfrom the support substrate 910.

In the present exemplary embodiment, the entering part 22 of the fusefilm 20 corresponds to a first entering part, and the entering part 14of the support substrate 10 corresponds to a second entering part. Inthe above description, both the entering parts 22 and the entering parts14 are formed, but it is not so limited. It is sufficient if at leastany one of the entering part 22 and the entering part 14 is formed. Evenin such a case, the fuse film 20 can be firmly bonded to the supportsubstrate 10.

The internal terminal groups 31 and 32 are bonded to the principalsurface 12 of the support substrate 10. As shown in FIG. 8, the internalterminal group 31 is a connection terminal connected to one end side inthe longitudinal direction of the fuse film 20, and the internalterminal group 32 is a connection terminal connected to the other endside in the longitudinal direction of the fuse film 20. The internalterminal group 31 includes three internal terminals 31 a, 31 b, and 31c, which are spaced apart from each other in the longitudinal direction,and includes internal terminals 31 d and 31 e that connect the threeinternal terminals 31 a, 31 b, and 31 c. Similarly, the internalterminal group 32 includes a plurality of internal terminals (internalterminals 32 a, 32 b, 32 c, 32 d, and 32 e). Since the internal terminalgroups 31 and 32 have the same configuration, the detailed configurationis described here taking the internal terminal group 31 as an example.

The internal terminals 31 a to 31 c of the internal terminal group 31are each arranged along the intersecting direction intersecting with thelongitudinal direction of the fuse film 20 (specifically, in the Ydirection orthogonal to the X direction, which is the longitudinaldirection, as shown in FIG. 8). Each of the internal terminals 31 a to31 c has the same width w, which is the same as the width w of the fusefilm 20. Further, the thickness of each of the internal terminals 31 ato 31 c is the same as the thickness t of the fuse film 20. The internalterminals 31 d and 31 c are provided on both sides of the fuse film 20along the longitudinal direction of the fuse film 20. The width and thethickness of the internal terminals 31 d and 31 e are the same as thewidth and the thickness of the internal terminals 31 a to 31 c.

In the present exemplary embodiment, each of the internal terminalgroups 31 and 32 has an entering part (corresponding to a third enteringpart) that enters the inside of the support substrate 10 from thebonding surface with the principal surface 12. The third entering parthas a similar shape as the entering part 22 of the fuse film 20.Therefore, the internal terminal groups 31 and 32 are also firmly bondedto the support substrate 10.

The support substrate 10 has an entering part (corresponding to a fourthentering part) that enters the inside of the internal terminal groups 31and 32 from the principal surface 12. The fourth entering part has asimilar shape as the entering part 14. Since the fourth entering part isformed in addition to the third entering part, it becomes possible tofurther strengthen the bonding between the internal terminal groups 31and 32 and the support substrate 10. It should be noted that, in theabove description, both the third entering part and the fourth enteringpart are formed, but it is not so limited. At least any one of the thirdentering part and the fourth entering part may be formed.

The overcoat 40 is a coating part that coats the position near thecenter in the longitudinal direction of the fuse film 20. Further, theovercoat 40 also coats i) the internal terminal 31 a located at theclosest position to the center in the longitudinal direction of theinternal terminal group 31 and ii) the internal terminal 32 a located atthe closest position to the center in the longitudinal direction of theinternal terminal group 32. The overcoat 40 is made of, for example, anorganic compound containing epoxy resin.

The external terminal 51 is electrically connected to a single internalterminal or a plurality of internal terminals in the internal terminalgroup 31 (the internal terminal 31 b and the internal terminal 31 c inFIG. 8) at one end in the longitudinal direction of the fuse film 20.The external terminal 52 is connected to a single internal terminal or aplurality of internal terminals in the internal terminal group 32 (theinternal terminal 32 b and the internal terminal 32 c in FIG. 8) atanother end in the longitudinal direction. The external terminals 51 and52 are made of, for example, silver.

It should be noted that, in the above description, the support substrate10 is a substrate made of an organic compound, but it is not so limited.For example, the support substrate 10 may be a substrate in which anorganic compound and an inorganic compound are combined. In such a case,the proportion of the organic compound is preferably larger than theproportion of the inorganic compound.

(Pulse Life Test)

The pulse life of the fuse 1 of the above-described present exemplaryembodiment is described in comparison with fuses according tocomparative examples 2 and 3. Here, a pulse life test was conducted onthe fuse 1 according to the present exemplary embodiment and fusesaccording to comparative examples 2 and 3 under the same conditions.

The fuse according to comparative example 2 is a fuse whose fuse film,which is made of silver, is formed on a principal surface of a polyimidesupport substrate by the vacuum evaporation method. The fuse accordingto comparative example 3 is a fuse whose fuse film is formed by dryingand firing in a blower furnace after printing a dispersion liquid, inwhich silver nanoparticles that are about 15 (nm) are dispersed, on aprincipal surface of a polyimide support substrate. The thicknesses ofthe support substrates and the fuse films of comparative examples 2 and3 are the same as the thickness (about 250 (μm)) of the supportsubstrate 10 and the thickness (about 0.25 (μm)) of the fuse film 20 ofthe present exemplary embodiment respectively.

FIG. 11 is a graph showing pulse life test results of the fuse 1according to the present exemplary embodiment and the fuses according tocomparative examples 2 and 3. A curve C1 in FIG. 11 shows the pulse lifeof the fuse 1 according to the present exemplary embodiment, the curveC2 shows the pulse life of the fuse according to comparative example 2in which the fuse film is formed by the vacuum evaporation method, andthe curve C3 shows the pulse life of the fuse according to comparativeexample 3 in which the fuse film is formed by firing by a blowerfurnace. As can be seen from the graph, the pulse life of the fuse ofcomparative example 2 is about 100 times and the pulse life of the fuseof comparative example 3 is about 120 times when, for example, thecurrent load factor is 90%. On the other hand, the pulse life of thefuse 1 of the present exemplary embodiment is about 4,500 times, and thepulse life is remarkably improved as compared with comparative examples2 and 3.

Next, the pulse life of the fuse 1 according to the present exemplaryembodiment and the pulse life of the fuse 900 according to comparativeexample 1, which was described with reference to FIG. 3, are describedwith reference to FIG. 12. FIG. 12 is a graph showing pulse life testresults of the fuse 1 according to the present exemplary embodiment andthe fuse 900 according to comparative example 1. The curve C1 in FIG. 12shows the pulse life of the fuse 1 according to the present exemplaryembodiment and the curve C4 shows the pulse life of the fuse 900according to comparative example 1. As can be seen from the graph, thepulse life of the fuse 1 according to the exemplary embodiment isremarkably improved as compared with the pulse life of the fuse 900according to comparative example 1.

(Peeling Test)

Next, the bonding strength of the fuse film 20 and the support substrate10 of the fuse 1 of the present exemplary embodiment is described incomparison with the above-described fuses according to comparativeexamples 2 and 3. Here, in order to ascertain the bonding strength, thetape peeling test was carried out under the same conditions for the fuse1 according to the present exemplary embodiment and the fuses accordingto comparative examples 2 and 3.

The tape peeling test was carried out in accordance with the 180 degreepeeling test method of “Testing methods of pressure-sensitive adhesivetapes and sheets” specified in JIS Z0237. That is, first, a test piece,which was a cut portion of the fuse, was adhered and fixed on a glasssubstrate, and tape was attached to the surface of the fuse film of thetest piece. Then, a glass substrate was fixed to a fixing jig and a testwas conducted to peel one end of the tape 180 degrees using a load cell.It should be noted that, for a test piece having a strong bonding force,a peeling test was conducted after bonding the fuse film and the tapewith an adhesive in advance.

The table 1 below shows the test results of the tape peeling test.

TABLE 1 Present exemplary Comparative Comparative embodiment example 2example 3 Initial peeling 3.1 0.37 1.12 strength (KN/m)

It should be noted that the initial peeling strength means the peelingstrength in an initial state before performing conduction through thefuse or bending the fuse.

As can be seen from the table 1, the peeling strengths of the fusesaccording to comparative examples 2 and 3 are 0.37 (KN/m) and 1.12(KN/m), respectively. On the other hand, the peeling strength of thefuse 1 according to the present exemplary embodiment is 3.1 (KN/m), andthe peeling strength is significantly larger than those of comparativeexamples 2 and 3. As a result, it was confirmed that the bondingstrength of the fuse 1 of the present exemplary embodiment was largerthan the bonding strengths of the fuses according to comparativeexamples 2 and 3. It should be noted that, from the results of the pulselife test and the peeling test described above, it was also confirmedthat there was a correlation between the pulse life and the peelingstrength shown in FIG. 11.

3. Fuse Production Method

An example of a method of producing the fuse 1 is described withreference to FIG. 13. FIG. 13 is a flowchart showing a productionprocess of the fuse 1. As shown in FIG. 13, the production process ofthe fuse 1 includes a liquid film forming process, a drying process, afuse film/internal terminal forming process, a cleaning process, afiring process, a post process, and an inspection process. Each processwill be described below.

(Liquid Film Forming Process: S102)

In the liquid film forming process S102, an ink film 110 that is aliquid film of a dispersion liquid, in which metal nanoparticles aredispersed in a solvent, is formed on a surface 102 (see FIG. 14) that isa principal surface of a composite substrate 100. Specifically, ink,which contains metal nanoparticles, having a predetermined thickness isformed on the entire surface 102 of the composite substrate 100 by usinga spin coater, which is not shown in figures.

As the metal nanoparticles, for example, silver nanoparticles are used.The average particle diameter of silver nanoparticles is 5 to 30 (nm),and it is about 15 (nm) herein. Further, the amount of silvernanoparticles contained in the ink (silver nanoink) is, for example,about 50 (wt %). It should be noted that the amount of the silvernanoparticles is not so limited, and may be, for example, 20 to 60 (wt%).

The solvent in the dispersion liquid is, for example, tetradecane whichis a type of hydrocarbon. Tetradecane is a low-boiling solvent, but thedispersion liquid may contain other solvents with a high boiling point.Further, the dispersion liquid contains a dispersant for dispersing themetal nanoparticles in a solvent, and the dispersant is composed of anorganic substance such as an aliphatic amine.

FIG. 14 is a schematic view showing the ink film 110 formed on thecomposite substrate 100. In the exemplary embodiment, the plurality ofink films 110, corresponding to the plurality of support substrates 10of the fuses 1, is formed on the composite substrate 100 and so a largeamount of fuses 1 can be manufactured. Here, the composite substrate 100is made of an organic compound (specifically, non-thermoplasticpolyimide). The composite substrate 100 has a thickness of about 250(μm) and a surface roughness Ra of about 0.05 (μm).

(Drying Process: S104)

In the drying process S104, the ink film 110 on the composite substrate100 is dried. Specifically, the ink film 110 is dried at a temperatureof, for example, about 70° C. for about one hour or less by using aheating blower furnace. As a result, a low boiling point solvent (forexample, a portion of tetradecane) in the ink film 110 evaporates, and adried ink film 110 (specifically, a nano silver ink film) having auniform thickness is formed on the composite substrate 100. As a result,the surface 102 of the composite substrate 100 is coated with the driedink film 110 and is isolated from the atmosphere.

(Fuse Film/Internal Terminal Forming Process: S106)

In the fuse film/internal terminal forming process, a laser irradiationapparatus radiates laser light onto the ink film 110 on the compositesubstrate 100 to form a fuse film and internal terminals. Beforedescribing the fuse film/internal terminal forming process, theconfiguration of the laser irradiation apparatus is described below.

(Configuration of Laser Irradiation Apparatus 200)

FIG. 15 is a schematic diagram showing an example of a configuration ofa laser irradiation apparatus 200. The laser irradiation apparatus 200includes a control part 210, a laser output part 220, an optical part230, a movable table 240, a table driving device 245, and a detectionpart 250.

The control part 210 controls the entire operation of the laserirradiation apparatus 200. For example, when the control part receivesthe CAD information on the shapes and positions of the fuse film and theinternal terminal from the personal computer, the control part 210radiates the laser light onto the ink film on the composite substrate100 at a relative scanning speed by controlling the movement of themovable table 240 and the radiation of the laser light. Further, thecontrol part 210 adjusts the scanning speed and the radiation intensityof the laser light.

The laser output part 220 includes a power supply 222 and a laseroscillator 224. The laser oscillator 224 continuously oscillates thelaser light in accordance with the output from the power supply 222.Here, the spot diameter φ (L) of the laser light is, for example, 10(μm). Further, the laser light is, for example, Nd-YAG laser lighthaving a wavelength of 1064 (nm) and an average radiation intensity from3.0×10⁴ to 5.0×10⁵ (W/cm²).

The optical part 230 includes a mirror 232, an optical filter 234, and alens 236. The mirror 232 adjusts the radiation direction of the laserlight. The optical filter 234 has a function of attenuating the lightamount of the laser light. The optical filter 234 is, for example, aNeutral Density (ND) filter. The lens 236 collects the laser lightattenuated by the optical filter 234.

By using the above-described optical filter 234, the selection range ofthe radiation condition (for example, the radiation intensity) of thelaser light is expanded. For example, in the case where the averageradiation intensity is controlled in a range from 3.0×10⁴ to 5.0×10⁵(W/cm²), oscillation of the laser light may become unstable, whichhinders the firing of the ink film, when the output of the power supply222 is set to a predetermined value or less. Because attenuation of thelight amount of the laser light is effective to cope with such aproblem, the optical filter 234 is used. Further, the optical filter 234is detachably mounted. Thus, an appropriate optical filter 234 can beselected and mounted from among optical filters having differentcharacteristics.

The movable table 240 is movable in the X-Y directions. The movabletable 240 includes a substrate sucking part, and sucks and holds thecomposite substrate 100. The table driving device 245 independentlymoves the movable table 240 in the X direction and the Y direction. Thedetection part 250 is, for example, a CCD camera, and detects theposition of the composite substrate 100 and the radiation state of thelaser light on the composite substrate 100.

The configuration of the laser irradiation apparatus 200 has beendescribed above. Next, details of the fuse film/inner terminal formingprocess S106 using the laser irradiation apparatus 200 are describedwith reference to FIGS. 16 and 17.

FIG. 16 is a flowchart showing details of a fuse film/internal terminalforming process. FIG. 17 is a diagram showing the composite substrate100 after formation of a fuse film/internal terminal. It should be notedthat FIG. 17 schematically shows a sub-assembly 118 including a fusefilm and an internal terminal corresponding to one fuse after formingthe fuse film/internal terminal.

In the fuse film forming process, first, the composite substrate 100having the ink film 110 formed on the surface 102 is sucked and fixed tothe movable table 240 (step S132). Next, the laser light irradiatescorners of the ink film 110 on the composite substrate 100 to formalignment marks 115 a, 115 b, and 115 c as shown in FIG. 17 (step S134).The shapes of the formed alignment marks 115 a to 115 c are, forexample, substantially cross shape. Here, the alignment marks 115 a to115 c are position adjustment marks for adjusting the forming positionfor forming the plurality of fuse films and internal terminals on thecomposite substrate 100.

Next, the three alignment marks 115 a to 115 c are read by the detectionpart 250, and the X direction and the Y direction of the compositesubstrate 100 are determined with reference to the position of the readalignment mark and the origin is also determined at the same time (stepS136). Here, the alignment mark 115 a is the origin.

(Heating Process of Ink Film 110: S138)

Next, the laser light irradiates the surface of the dried ink film 110to heat the ink film 110 (step S138). On this occasion, the portion thatis irradiated with the laser light is specified on the basis of theposition of the alignment mark 115 a (origin). That is, the control part210 receives the CAD information relating to i) the shapes of the fusefilm and the internal terminal and ii) the positions of the fuse filmand the internal terminal based on the position of the alignment mark115 a from the personal computer, and controls the movement of themovable table 240 and the radiation of the laser light. For example, thecontrol part 210 radiates the laser light substantially perpendicularlyto the surface of the ink film 110 at a scanning speed of about 3 to 90(mm/sec).

In the present exemplary embodiment, the ink film 110 is heated so as tovaporize the high boiling point solvent and dispersant included in theink film 110. Specifically, the ink film 110, which is irradiated by thelaser light, is mainly composed of a solvent having a high boilingpoint, a dispersant, and silver nanoparticles. Here, since the silvernanoparticles have an average particle diameter of about 15 (nm) andhave an absorption characteristic of absorbing the laser light having awavelength of 1064 (nm), they absorb light (plasmon absorption) togenerate heat. As a result, when the temperature of the silvernanoparticles rises to, for example, 500° C., (a portion of) thehigh-boiling solvent and the dispersant are vaporized. For example,solvents and dispersants evaporate or gasify (oxidize). Then, as thedispersant is vaporized, the dispersant and the silver nanoparticlesseparate from each other.

The silver nanoparticles that separated from the dispersant are in abare state and the activity of the surface of the silver nanoparticlesis enhanced. Then, the silver nanoparticles melt, and some of the silvernanoparticles are sintered to form silver particles. The molten silvernanoparticles or the sintered silver particles transfer heat to thesurface 102 of the contacting composite substrate 100, which is anon-thermoplastic polyimide substrate, to heat the surface 102. Thesurface 102 is heated to a temperature close to about 500° C. Further,the surface 102 is substantially isolated from the atmosphere by silvernanoparticles (or silver particles) and a dispersant located on thesurface 102.

The heated surface 102 exceeds (specifically, at a temperature lowerthan 600° C.) the glass transition temperature (about 420° C.) of thecomposite substrate 100 and is softened or melted in a statesubstantially isolated from the atmosphere.

Here, the reason for causing the surface 102 to be substantiallyisolated from the atmosphere is to prevent occurrence of an undesirablephenomenon such as easy carbonization of the surface 102 when thesurface 102 is in contact with the atmosphere. Further, the temperatureof the heated surface 102 is preferably controlled to be higher than theglass transition temperature (about 420° C.) and not more than 600° C.When the temperature of the surface 102 reaches a temperature greatlyexceeding 500° C. (for example, 600° C. to 700° C.), carbonization ofthe surface 102 proceeds, and so the surface 102 cannot be softened ormelted sufficiently. Similarly, when the temperature of the surface 102does not reach the glass transition temperature, the surface 102 alsocannot be sufficiently softened or melted.

In the present exemplary embodiment, the laser light irradiates the inkfilm 110 only once to heat the ink film 110. Therefore, it is preferablethat the radiation intensity of the laser light is large. But if theradiation intensity is excessively large, the ink film 110 is blown away(so-called ablation) when the ink film 110 on the surface 102 isirradiated with the laser light, and the surface 102 has a possibilityof being exposed to the atmosphere and being carbonized.

On the other hand, when the radiation intensity of the laser light isset lower and the laser light is radiated a plurality of times, thefollowing problems occur. The light absorption reaction of silvernanoparticles in the ink film 110 occurs in the surface layer of the inkfilm 110. Therefore, an absorption exothermic reaction occurs in thesurface layer of the ink film 110 with the first irradiation by thelaser light, and so carbonization and hardening of the dispersant in thesurface layer and sintering of the silver nanoparticles occur.Thereafter, when the second irradiation is performed, the carbonized orhardened dispersant or the like becomes an obstacle and the laser lightdoes not sufficiently reach the unsintered silver nanoparticles existingin the lower portion of the surface layer, and so the surface 102 of thecomposite substrate 100 cannot be heated sufficiently. In addition,since gas and the like generated in the lower portion of the surfacelayer are blocked by the surface layer and cannot be sufficientlydischarged to the atmosphere, it becomes difficult to manage physicalproperty values such as resistivity of the ink film 110.

From the above-described result of examination on the radiationintensity of the laser light, the laser light is Nd-YAG laser lighthaving a wavelength of 1064 (nm) and an average radiation intensity from3.0×10⁴ to 5.0×10⁵ (W/cm²) as described above. But it is not so limited,and the laser light may be Nd-YAG laser light having a wavelength of 532(nm) and an average radiation intensity of 2.0×10³ to 7.0×10⁴ (W/cm²).Because silver nanoink has a higher absorptance of the harmonic laserlight having a wavelength of 532 (nm) than of the laser light having awavelength of 1064 (nm), the average radiation intensity of the laserlight having a wavelength of 532 (nm) is set lower.

In order to appropriately soften or melt the surface 102 by irradiatingthe ink film 110 with the laser light, it is necessary to control thescanning speed of the laser light in addition to the radiation intensityof the laser light. For example, when the scanning speed of the laserlight exceeds 90 (mm/s), the surface 102 could not be softened or meltedsufficiently. Therefore, in the present exemplary embodiment, thescanning speed of the laser light is set to 3 to 90 (mm/s). It should benoted that, regarding the setting of the radiation intensity and thescanning speed of the laser light, it is preferable to consider thethickness of the ink film 110 and the spot diameter of the laser lightparticularly.

Here, the present exemplary embodiment is described by applying theknowledge of thermodynamics. In the system in which the laser lightirradiates the surface of the ink film 110 to heat the surface, theaverage distance L(L) in the thickness direction of the ink film 110that the heat reaches is expressed by the following expression (3).L(L)=K ₁·(κ_(i))^(α)·τ^(β)  (3)

It should be noted that κ_(i) is the average thermal diffusivity in thethickness direction of the ink film 110, τ is the radiation time of thetypical laser light, α and β are predetermined numbers which are α>0,β>0, and K₁ is a proportional constant.

Further, when the spot diameter of the laser light to be radiated isφ(L) and the relative scanning speed of the laser light is V(L), therepresentative radiation time τ of the laser light according to thepresent exemplary embodiment for irradiating the ink film 110 with thelaser light in the continuous oscillation mode is expressed by thefollowing expression (4).τ=K ₂·ϕ(L)/V(L)  (4)

It should be noted that K₂ is a correction coefficient related to theshape and the like of the laser light.

When expression (4) is substituted in expression (3), the followingexpression (5) is obtained.L(L)=K ₁·(κ_(i))^(α)·(K ₂·ϕ(L)/V(L))^(β)  (5)

According to the expression (5), the distance L(L) that heat reaches isdetermined by each factor of κ_(i), φ(L), and V(L), which means thatthere is a combination of each factor value. That is, when the thermaldiffusivity κ_(i) and the spot diameter φ(L) are fixed values, thedistance L(L) is considered to be determined by the scanning speed V(L).In the present exemplary embodiment, if it is considered that thedistance L(L) represents the distance from the surface of the ink film110 to the surface 102 of the composite substrate 100 (thickness forheating the ink film 110), when the thickness of the ink film 110 andthe average thermal diffusivity κ_(i) are fixed values, it can beconsidered that the scanning speed V(L) needs to be selected accordingto the spot diameter φ(L). Further, as a result of observing the changein the state of softening or melting the surface 102 of the compositesubstrate 100 depending on the thickness t(L) of the ink film 110 bychanging the spot diameter φ(L) and the scanning speed V(L), it wasfound that the distance L(L) has a strong correlation with the thicknesst(L). That is, it can be considered that the average distance L(L) thatthe heat reaches in the thickness direction of the ink film 110 isindicative of the thickness t(L).

It should be noted that, when the thickness of the ink film 110 waslarger than about 3.0 (μm), it is necessary to heat the ink film 110with extremely low scanning speed of the laser light, and so it wasjudged that it was not practical in the present exemplary embodiment. Onthe other hand, when the thickness was smaller than about 0.1 (μm), thesurface 102 of the composite substrate 100 could not be stably softenedor melted even when the scanning speed of the laser light was increased.

In the present exemplary embodiment, in order to stably soften or meltthe surface 102 while preventing the carbonization and excessivedeformation of the surface 102 by heating the surface 102 of thecomposite substrate 100 in a predetermined temperature range, theheating conditions of the ink film 110, the metal nanoparticles, thesolvent, the dispersant, and the physical properties, shape, size, andthe like of composite substrate are considered.

(Fusion Process of Metal Nanoparticle and Surface 102: S140)

After irradiating the ink film 110 on the surface 102 with the laserlight in step S138, the melted or sintered metal nanoparticles in theink film 110 and the softened or melted surface 102 are fused to eachother by, for example, leaving them for a predetermined time (StepS140). As a result, as shown in FIG. 18, the fuse film 120 and theinternal terminal groups 130 are formed on the surface 102. That is, theportions of the ink film 110 heated by being irradiated with the laserlight become the fuse film 120 and the internal terminal groups 130.

Here, the fusion state of the metal nanoparticles and the surface 102 isdescribed. The melted or sintered metal nanoparticles in the ink film110 come into contact with the softened or melted surface 102 to form amutually fused bonding interface. That is, the surface tensions of themetal nanoparticles and the surface 102 interact with each other, and abonding interface where the metal nanoparticles and the surface 102 arewetted with each other is freely formed. By forming a bonding interface,the fuse film 120 and the internal terminal groups 130 are formed on thesurface 102.

Specifically, the metal nanoparticles are deformed to, for example, aspherical surface shape due to the surface tension, and the surface 102having a surface tension larger than that of the metal nanoparticlesforms a bonding interface by being deformed to envelop the sphericallydeformed metal nanoparticles due to the surface tension.

The metal nanoparticles constituting the fuse film 120 and the internalterminal groups 130 form an entering part (specifically, the enteringpart 22 shown in FIG. 9) extending into the inside of the compositesubstrate 100 at the bonding interface and entering into the inside ofthe composite substrate 100. The entering part has, for example, a hookshape and is engaged with the inside of the composite substrate 100. Thewidth of the distal end of the hook-shaped entering part is larger thanthe width of the root part side of the entering part. By forming such anentering part, it is possible to firmly bond the fuse film 120 to thecomposite substrate 100.

FIG. 18 is a diagram showing a formation state of the fuse film 120 andinternal terminal groups 130. The fuse film 120 and the internalterminal groups 130 constituting one sub-assembly 118 extend linearly,and they are connected to the fuse film 120 and the internal terminalgroups 130 of the other sub-assembly 118. A portion of the fuse film 120and the internal terminal groups 130 protruding from the sub-assembly118 is cut off when the sub-assembly 118 is cut out from the compositesubstrate 100. It should be noted that the fuse film 120 and theinternal terminal groups 130 may be formed in a manner that the fusefilm 120 and the internal terminal groups 130 do not protrude from thesub-assembly 118, unlike in FIG. 18.

As can be seen from FIG. 18, the fuse film 120 has a linear shapeextending in the X direction. The width w of the fuse film 120 is, forexample, 10 (μm) which is approximately the same size as the spotdiameter φ(L) of the laser light. The thickness of the fuse film 120 is,for example, 0.25 (μm). The internal terminal groups 130 arerespectively connected to the fuse film 120 at both ends in thelongitudinal direction of the sub-assembly 118 of the fuse film 120.Each of the two internal terminal groups 130 includes three internalterminals 131 a to 131 c and internal terminals 132 a to 132 c havingthe same shape. Further, the internal terminal groups 130 includeinternal terminals 131 d and 131 e for connecting the internal terminals131 a to 131 c, which are spaced apart from each other, and internalterminals 132 d and 132 e for connecting the internal terminals 132 a to132 c.

Each of the internal terminals 131 a to 131 e and 132 a to 132 e of theinternal terminal groups 130 is formed under the same radiationconditions of the laser light as the time when the fuse film 120 isformed. Therefore, the width w of each internal terminal (the internalterminal 131 a is described as an example) of the internal terminalgroups 130 has the same size as the width w of the fuse film 120.Further, the thickness of the internal terminal 131 a is also the sameas the thickness of the fuse film 120. Then, the internal terminalgroups 130 are firmly bonded to the composite substrate 100 by theentering part (specifically, the third entering part and the fourthentering part described above) like the fuse film 120. It should benoted that the width and the thickness of each internal terminal of theinternal terminal groups 130 may be different from the width and thethickness of the fuse film 120. In addition, the radiation condition ofthe laser light at the time of forming the internal terminal groups 130may be different from the radiation condition at the time of forming thefuse film 120.

In the present exemplary embodiment, because the fuse film 120 and theinternal terminal groups 130 are formed in one process, the positionalaccuracy of the internal terminal groups 130 with respect to the fusefilm 120 can be improved as compared with the case where the fuse film120 and the internal terminal groups 130 are formed in separateprocesses. Further, the process at the time of producing is simplified,and low cost can be easily realized.

In the present exemplary embodiment, the linear fuse film 120 and theinternal terminal groups 130 having the width corresponding to the spotdiameter of the laser light is formed by scanning the ink film 110 oncewith the laser light. Furthermore, the fuse film 120 and the internalterminal groups 130 having the length corresponding to the scanningwidth of the laser light are formed. As a result, a large amount of thefuse films 120 and the internal terminal groups 130 can be formed in ashort time.

The thickness (the second thickness) of the ink film 110 afterirradiation of the laser light is smaller than the thickness (the firstthickness) of the ink film 110 before irradiation with the laser light.Because the correspondence between the first thickness and the secondthickness has been previously analyzed by experiments or the like, inthe process of forming the ink film 110 in step S102 described above,the ink film 110 is formed by adjusting the first thickness on the basisof the correspondence between the first thickness and the secondthickness. Thus, the fuse film 120 and the internal terminal groups 130can be appropriately controlled to be a desired thickness.

FIG. 19 is a graph showing the relationship between the thickness t(i)of the ink film 110 before laser irradiation and the thickness t of thefuse film 120 after irradiation. Here, the ink film 110 is an ink filmcontaining silver nanoparticles and is formed on a polyimide substrate.As can be seen from the graph, the thickness t(i) of the ink film 110and the thickness t of the fuse film 120 are in proportion to eachother, and the thickness t after irradiation can be controlled bycontrolling the thickness t(i) before irradiation.

It should be noted that similar results were obtained in experimentsusing an inkjet instead of a spin coater. In other printing methods suchas flexographic printing and gravure printing, it was confirmed that thethickness t of the fuse film 120 after irradiation could be controlledby controlling the thickness t(i) of the ink film 110.

FIG. 20 is a graph showing the relationship between the spot diameterφ(L) of the laser light and the width w of the fuse film 120. As shownin the graph, the width w of the fuse film 120 after irradiation and thespot diameter φ(L) are proportional to each other. It should be notedthat the spot diameter φ(L) was obtained by measuring with a beamprofiler, or measuring the shape of the trace processed by actuallyirradiating the substrate with the laser light.

In the present exemplary embodiment, the control part 210 may radiatethe laser light onto the ink film 110 while adjusting at least one ofthe radiation speed and the radiation intensity of the laser lightaccording to the thickness of the ink film 110. As a result, the fusefilm 120 and the internal terminal groups 130 having a desired thicknesscan be formed even when the set value of the thickness of the ink film110 is changed. Further, the fuse film 120 and the internal terminalgroups 130 can be firmly bonded to the composite substrate 100.

In the present exemplary embodiment, as described above, the laser lightoscillated from the laser oscillator 224 is attenuated by the opticalfilter 234 for attenuation, and the attenuated laser light is applied tothe ink film 110. Oscillation of the laser light tends to be unstablewhen the output of the power supply 222 is made smaller than apredetermined value. It is possible to secure a desired radiationintensity by attenuating the light amount with the optical filter 234instead of making the output of the power supply 222 smaller thannecessary. This makes it possible to suppress the instability ofoscillation of the laser light, and so the ink film 110 (the fuse film120 and the internal terminal groups 130) after irradiation can beadequately bonded to the composite substrate 100.

It should be noted that, the linear fuse film 120 is formed as in theabove description, but it is not so limited, and may be formed as acurved fuse film, for example. The curved fuse film can be formed byproviding a galvanometer mirror on the optical part 230 and scanningwith the laser light. Alternatively, a fuse film combining a linear fusefilm and a curved fuse film may be formed. As a result, a fuse havingthe fuse film 120 having various shapes can be produced.

(Cleaning Process S108)

Returning to FIG. 13, in the cleaning process, the ink on the ink film110 that is not irradiated with the laser light is washed away and aportion on the ink film 110 irradiated with the laser light is dried. Itshould be noted that, as a cleaning method, for example, ultrasoniccleaning with an isopropyl alcohol solution is used.

After cleaning, the electric resistance R between adjacent internalterminals (for example, the internal terminal 131 a and the internalterminal 132 a) may be measured. By using the measured electricresistance R, the resistivity ρ can be obtained with the followingexpression (6). It should be noted that, for the measurement of theelectric resistance R, a known four-terminal method was used.ρ=R·t·w/L  (6)(Firing Process S110)

In the firing process, the composite substrate 100 on which the fusefilm 120 and the internal terminal groups 130 are formed is fired atabout 250° C. for 1 hour by using, for example, a blower furnace. Afterfiring, the electric resistance R between the adjacent terminals may bemeasured to obtain the resistivity p. From the measurement results, thevariation in the resistivity after the firing process is improved overthe variation in the resistivity after the cleaning process.

In particular, when the spot shape of the laser light is circular, thesintering of the metal nanoparticles at both end portions of thescanning area of the laser light is insufficient, and the resistivity atboth end portions tends to be high. On the other hand, it was foundthat, by performing the firing, the variation in the resistivity wasreduced and sintering of the metal nanoparticles was sufficientlyperformed at both end portions. In the present exemplary embodiment, theresistivity ρ after firing was 4.5 (μΩcm).

(Post Process S112)

In the post process, the overcoat and the external terminal are mainlyformed. In the following, details of the post process are described withreference to FIG. 21.

FIG. 21 is a flowchart showing details of the post process. First, asshown in FIG. 22, an overcoat 140 is formed on the sub-assembly 118(step S152). The overcoat 140 is formed by determining the positions ofthe respective sub-assemblies 118 on the composite substrate 100 on thebasis of the aforementioned origin (the position of the alignment mark115 a).

FIG. 22 is a diagram showing a formation state of the overcoat 140 onthe sub-assembly 118. The overcoat 140 is formed to coat the positionnear the center in the longitudinal direction of the fuse film 120. Theovercoat 140 is mainly made of silicone resin. The overcoat 140 isformed using, for example, screen printing. Specifically, the overcoat140 is formed by heating and curing the resin at a predeterminedtemperature after printing.

Returning to FIG. 21, the sub-assembly 118 having the overcoat 140formed thereon is cut out from the composite substrate 100 (step S154).Next, as shown in FIG. 23, external terminals 151 and 152 connected tothe internal terminals are formed on both end portions in thelongitudinal direction of the sub-assembly 118 (step S156).

FIG. 23 is a diagram showing a formation state of the external terminals151 and 152. The external terminals 151 and 152 are formed to connectwith portions of the internal terminal groups 130 that are not coated bythe overcoat 140. The external terminals 151 and 152 are mainly composedof silver. The external terminals 151 and 152 are formed by printing andforming a silver paste that is an organic solvent, in which silverparticles are dispersed, by using a screen printing technique or adipping technique and then firing the silver paste under predeterminedheating conditions.

In the present exemplary embodiment, the bonding strength between theinternal terminal groups 130 and the external terminals 151 and 152 ishigh. This is because i) the electrical and mechanical bonding betweenthe silver nanoparticles constituting the internal terminal groups 130and the silver particles constituting the external terminals 151 and 152is secured and ii) the mechanical bonding between the organic solvent,in which silver nanoparticles are dispersed, and the organic solvent, inwhich silver particles are dispersed, is secured.

By forming the external terminals 151 and 152, the product-type fuse 1is formed. Returning to FIG. 21, the surface of the overcoat 140 isstamped with a seal as shown in FIG. 24 (step S158). FIG. 24 is adiagram for describing the stamping of a seal on the overcoat 140. Forexample, a character is stamped on the surface of the overcoat 140. Itshould be noted that, after stamping a seal the overcoat 140, Ni platingor Sn plating may be applied to the external terminals 151 and 152.

(Inspection Process S114)

Returning to FIG. 13, in the inspection process, the resistance and thelike of the fuse 1 are inspected. After the inspection, the fuse 1 ispacked and shipped. A series of production processes of the fuse 1according to the present exemplary embodiment is completed.

In the production method of the fuse 1 described above, after the inkfilm 110 containing the metal nanoparticles is formed on the substrate,the fuse film 120 is formed by irradiating the ink film 110 with thelaser light. In such a case, it is possible to inexpensively produce alarge amount of the fuses 1 having a fine thin fuse film without usingpatterned ground processing, a patterned mask, or the like of a fusefilm.

Further, according to the above-described production method of the fuse1, the fuse film 120 and the internal terminal groups 130 formed on thepolyimide composite substrate 100 by heating the ink film 110 with thelaser light are reliably bonded to the surface of the compositesubstrate 100 by forming the entering parts respectively. The bondingstrength at the bonding interface is larger than the misalignment forceF at the bonding interface generated due to the difference in linearexpansion coefficients among the composite substrate 100 (the supportsubstrate after production), the fuse film 120, and the internalterminal groups 130 when the temperature of the fuse 1 changes due to aheat cycle test or the like, and so peeling at the bonding interface canbe prevented. As a result, it is possible to improve the pulse life andthe heat cycle reliability of the fuse 1.

It should be noted that, in the above-described production method of thefuse 1, step S102 corresponds to a liquid film forming step, step S138corresponds to a heating step, and step S140 corresponds to a fuse filmforming step.

4. Variation

In the above description, the spin coater is used to form the ink film110 containing metal nanoparticles on the entire surface 102 of thecomposite substrate 100, but it is not so limited. For example, an inkfilm may be formed on a portion of the surface 102 where the fuse film120 is to be formed, using an ink jet printer.

Further, in the above description, the metal nanoparticles are silvernanoparticles, but it is not so limited. For example, the metalnanoparticles may be other metal nanoparticles such as coppernanoparticles, gold nanoparticles, and nickel nanoparticles.Furthermore, in the above description, the average particle diameter ofmetal nanoparticles is about 15 (nm), but it is not so limited. Forexample, the average particle size of metal nanoparticles may be, forexample, 3 (nm) or 50 (nm).

Moreover, in the above description, the support substrate 10 is anon-thermoplastic polyimide substrate, but it is not so limited. Forexample, the support substrate 10 may be any of a thermoplasticpolyimide substrate, a thermosetting polyimide substrate, apolyetheretherketone (PEEK) substrate, or a substrate made of otherorganic materials.

Further, in the above description, the internal terminal groups 31 and32 include the internal terminals 31 d, 31 e, 32 d, and 32 erespectively connecting the internal terminals 31 a to 31 c and 32 a to32 c, but is not so limited and the internal terminal groups 31 and 32do not have to respectively include 31 d, 31 e, 32 d, and 32 e.

Furthermore, in the above description, the external terminals 51 and 52are respectively in contact with the internal terminals of the internalterminal groups 31 and 32, so as to be electrically connected, but it isnot so limited. For example, the external terminals 51 and 52 may beelectrically connected to the internal terminals through flat plate-likeintermediate terminals provided between the external terminals 51 and 52and the internal terminal groups 31 and 32.

Moreover, in the above description, the laser light is Nd-YAG laserlight having a wavelength of 1064 (nm) and an average radiationintensity of 3.0×10⁴ to 5.0×10⁵ (W/cm²), or Nd-YAG laser light having awavelength of 532 (nm) and an average radiation intensity of 2.0×10³ to7.0×10⁴ (W/cm²), but it is not so limited. For example, the laser lightmay be titanium sapphire laser light having a wavelength of 800 (run)that the metal nanoparticles have a plasmon absorption band. Further,the magnitude of the average radiation intensity of the laser light maybe adjusted according to the wavelength of the laser light.

Furthermore, in the above description, the laser light is radiated inthe continuous oscillation mode, but it is not so limited and the laserlight may be radiated in, for example, the pulse oscillation mode.Moreover, in the above description, the scanning speed of the laserlight is set to 3 to 90 (mm/s), but it is not so limited.

Further, in the above description, the spot shape of the laser light iscircular, but it is not so limited. For example, the spot shape of thelaser light may be any of an elliptical shape, a square shape, and arectangular shape. When the spot shape is square or rectangular, it ispossible to sinter across substantially the entire radiation width ofthe laser light. Furthermore, in the above description, the diameter ofthe laser light having the circular spot shape is 10 (μm), but it is notso limited. The diameter of the laser light may be adjusted according tothe wavelength and the radiation intensity of the laser light.

Moreover, in the above description, one linear fuse film 20 is formed,but it is not so limited. For example, a curved fuse film 20 may beformed and a plurality of fuse films 20 may be formed. Further, in theabove description, the thicknesses of the fuse 20 and the internalterminal groups 31 and 32 are set to 0.1 (μm) to 3.0 (μm), but they arenot so limited.

Furthermore, in the above description, the ink film is heated byirradiating the ink film 110 with the laser light, but it is not solimited. For example, the ink film 110 may be heated by known microwaveheating or induction heating. However, in order to intensively heatingthe ink film 110 in a short time to prevent deformation of the compositesubstrate 100 (a support substrate), a method of radiating the laserlight is effective.

5. Configuration of Circuit Board

With reference to FIG. 25 and FIG. 26, a configuration of a circuitboard 500 according to one exemplary embodiment of the present inventionis described.

The circuit board 500 of the present exemplary embodiment is a flexiblecircuit board having flexibility to be incorporated in a movable device(for example, a foldable portable terminal that can be opened andclosed). The circuit board 500 is provided, for example, inside of ahinge part of the portable terminal, and is bent in conjunction with theopening and closing of the portable terminal. Further, the circuit board500 may be provided on a wearable terminal that is weight-reduced anddownsized to be worn by a user.

When the above-described circuit board incorporated in the movabledevice is repeatedly bent, the following problems occur. That is, whenthe circuit board is repeatedly bent, cracks are generated in thecircuit part (a circuit pattern) bonded to the substrate of the circuitboard and the circuit part is divided, which increases the resistancevalue of the circuit part. Further, when the circuit board is repeatedlybent, the circuit part is peeled from the substrate and the circuitboard may be damaged. On the other hand, the circuit board 500 accordingto the present exemplary embodiment is configured to be able to suppressthe occurrence of the above-described problems even when it isrepeatedly bent.

FIG. 25 is a schematic cross-sectional view of the circuit board 500according to one exemplary embodiment. FIG. 26 is a schematic planarview of the circuit board 500. As shown in FIG. 25 and FIG. 26, thecircuit board 500 includes a substrate 510, a circuit part 520,terminals 530, and a coating part 540.

The substrate 510 supports the circuit part 520 and the terminals 530.The substrate 510 is made of, for example, an organic compound havingexcellent flexibility. Here, the substrate 510 is a non-thermoplasticpolyimide resin substrate. The thickness of the substrate 510 is about250 (μm).

The circuit part 520 is a circuit pattern made of conductor and isbonded to a principal surface 512 of the substrate 510. Like the fusefilm 20 of the fuse 1 described above, the circuit part 520 is formed onthe principal surface 512 by irradiating the ink film containing metalnanoparticles with the laser light. For example, silver nanoparticlesare used as metal nanoparticles. The thickness of the circuit part 520is about 0.25 (μm), and the length of the circuit part 520 is about 10(mm).

The circuit part 520 includes a circuit side entering part that entersthe inside of the substrate 510 from a bonding surface with theprincipal surface 512. A plurality of circuit side entering parts isdispersedly formed by fusions of metal nanoparticles with the principalsurface 512 of the substrate 510, the metal nanoparticles being meltedor sintered by being irradiated with the laser light at the time offorming the circuit part 520. The circuit side entering part is engagedwith the inside of the substrate 510. The shape of the circuit sideentering part is the same as the shape of the above-described enteringpart 22 of the fuse film 20, and so a detailed description thereof willbe omitted. Because the circuit side entering part is formed, thecircuit part 520 is firmly bonded to the substrate 510, and the circuitpart 520 becomes resistant to being peeled from the principal surface512 of the substrate 510. Further, because the circuit part 520 isfirmly bonded to the substrate 510, cracks are less likely to begenerated in the circuit part 520, and so an increase in the resistancevalue of the circuit part 520 can be suppressed.

Here, the mechanism of peeling the circuit part from the substrate isdescribed by taking the circuit board of comparative example 4 as anexample. The circuit part of the circuit board according to comparativeexample 4 is formed by firing an ink film in a blower furnace withoutirradiating the ink film corresponding to the circuit part with thelaser light. Therefore, a circuit side entering part is not formed inthe circuit board of comparative example 4 unlike the circuit part 520of the present exemplary embodiment. The circuit board according tocomparative example 4 is bent in accordance with the rotation of thedevice in which a rotating substrate is incorporated. When the circuitboard is repeatedly bent, a misalignment force is repeatedly generatedin a direction along the bonding interface at the bonding interfacebetween the circuit part and the substrate, and so the circuit part isdeviated from the principal surface of the substrate due to themisalignment force and the circuit part peels from the principal surfaceof the substrate. It should be noted that the misalignment force acts asa compressive force on a member having a large radius of curvature andacts as a tensile force on a member having a small radius of curvatureof the two bonded members at the bonding interface. That is, in a bentstate where the radius of curvature of the substrate is larger than theradius of curvature of the circuit part, a tensile force acts on thebonding surface of the circuit part and a compressive force acts on theprincipal surface of the substrate.

On the other hand, in the present exemplary embodiment, because thecircuit side entering part is formed in the circuit part 520, thecircuit part 520 is bonded to the substrate 510 by the bonding forcethat is larger than the above-mentioned misalignment force. Therefore,since it is hard for the circuit part 520 to be displaced from thesubstrate 510 even when the circuit board 500 is repeatedly bent,peeling of the circuit part 520 from the substrate 510 can effectivelybe prevented.

The substrate 510 also includes a substrate side entering part thatenters the inside of the circuit part 520 from the principal surface512. A plurality of substrate side entering parts is dispersedly formedand is engaged with the inside of the circuit part 520. In this way,since the substrate side entering part is formed in addition to thecircuit side entering part, it is possible to further strengthen thebonding between the circuit part 520 and the substrate 510.

The terminals 530 are formed at each end of the circuit part 520 in thelongitudinal direction and are bonded with the principal surface 512 ofthe substrate 510. The terminals 530 are connected to the both ends inthe longitudinal direction of the circuit part 520. Here, the terminals530 are made of silver like the circuit part 520. The terminal 530 maybe formed by irradiating the ink film with the laser light like thecircuit part 520, or may be formed by screen printing or the like.Because an entering part is formed like the circuit part 520 when theterminal 530 is made by irradiating the ink film with the laser light,the terminals 530 are firmly bonded to the substrate 510.

The coating part 540 coats the circuit part 520 and the terminals 530.The coating part 540 is, for example, a laminate film having high gasbarrier properties.

As described above, in the circuit board 500 according to the presentexemplary embodiment, the circuit part 520 is firmly bonded to thesubstrate 510 because the circuit side entering part and the substrateside entering part are formed. As a result, even when the circuit board500 is repeatedly bent, an increase in the resistance value of thecircuit part 520 can be suppressed, and the circuit part 520 is lesslikely to be peeled from the substrate 510.

Here, the relationship between i) the resistance value and the peelingstrength before the bending test, in which the circuit board 500according to the present embodiment is repeatedly bent a predeterminednumber of times (here, 3000 times), and ii) the resistance value and thepeeling strength after the bending test is described in comparison withthe circuit board according to comparative examples 4 and 5. The circuitboard according to comparative example 4 is a circuit board in which asilver circuit part is formed on the principal surface of a polyimidesubstrate by vacuum deposition. The fuse according to comparativeexample 5 is a circuit board whose circuit part is formed by printing adispersion liquid, in which silver nanoparticles with a size of about 15(nm) are dispersed, on the principal surface of a polyimide substrateand drying and firing the substrate in a blower furnace.

The following table 2 shows the resistance values of the circuit boardbefore the bending test and after the bending test.

TABLE 2 Resistance before Resistance after bending bending (Ω) (Ω)Present exemplary embodiment 7.56 7.60 Comparative example 4 0.25 0.43Comparative example 5 0.61 2.44

As can be seen from the table 2, the resistance values of the circuitboards according to comparative examples 4 and 5 after the bending testare greatly increased as compared with the resistance values before thebending test. On the other hand, the resistance value of the circuitboard 500 according to the present exemplary embodiment after thebending test is only slightly increased from the resistance value beforethe bending test, and the resistance hardly increases.

The following table 3 shows the peeling strength of the circuit partbefore the bending test and after the bending test.

TABLE 3 Peeling strength Peeling strength before bending after bending(KN/m) (KN/m) Present exemplary embodiment 3.1 3.1 Comparative example 40.37 0.08 Comparative example 5 1.12 0.80

As can be seen from the table 3, the peeling strengths of the circuitparts of the circuit boards according to comparative examples 4 and 5after the bending test is smaller than the peeling strength before thebending test. On the other hand, the peeling strength of the circuitpart 520 of the circuit board 500 according to the present exemplaryembodiment after the bending test is the same as the peeling strengthbefore the bending test. As a result, it was confirmed that the circuitpart 520 according to the present exemplary embodiment was not peeledfrom the substrate 510.

It should be noted that the bending test of the circuit board wascarried out in accordance with the “Test method of folding endurance”specified in JIS P8115. As a test condition, the bending radius that thecircuit board bends is 6 (mm), the bending angle is 90 (degrees), thebending speed is 120 (reciprocating/minute), and the tensile load is 1(N). Further, the lengths in the longitudinal direction of the circuitboard 500 used for the bending test and the circuit boards ofcomparative examples 4 and 5 are each about 20 (mm). It should be notedthat the width (Y direction in FIG. 25) of the circuit board 500 usedfor the bending test was 300 (μm), whereas the widths of the circuitboards of comparative examples 4 and 5 were each 10 (mm).

In the present exemplary embodiment, the circuit side entering part ofthe circuit part 520 corresponds to a first entering part, and thesubstrate side entering part of the substrate 510 corresponds to asecond entering part. In the above description, both of the circuit sideentering part and the substrate side entering part are formed, but it isnot so limited. At least any one of the circuit side entering part andthe substrate side entering part may be formed. In such a case, thecircuit part 520 can be firmly bonded to the substrate 510.

In the above description, the substrate 510 is a substrate made of anorganic compound, but it is not so limited. For example, the substrate510 may be a substrate in which an organic compound and an inorganiccompound are combined. In such a case, it is desirable that theproportion of the organic compound is larger than the proportion of theinorganic compound.

6. Circuit Board Production Method

With reference to FIG. 27, an example of a production method of thecircuit board 500 is described. FIG. 27 is a flowchart showing aproduction process of the circuit board 500. It should be noted that thesame process as the production process of the fuse 1 shown in FIG. 13 isbriefly described here because a similar process is performed therein.

In the liquid film forming process S202, an ink film that is a liquidfilm of a dispersion liquid, in which metal nanoparticles (silvernanoparticles) are dispersed in a solvent, is formed on the principalsurface 512 of a polyimide substrate 510. After the ink film is formed,the ink film may be dried.

Next, in the circuit part/terminal forming process S204, the laser lightirradiates the ink film to form the circuit part 520. At this time, thesurface of the ink film is irradiated with the laser light to heat theink film so as to vaporize the solvent constituting the ink film. As aresult, silver nanoparticles in the ink film melt, and some of thesilver nanoparticles are sintered together to form silver particles.Further, heat is transferred from the silver nanoparticles to theprincipal surface 512 of the substrate 510, and the principal surface512 is heated. As a result, the principal surface 512 softens or meltssubstantially in a state of being isolated from the atmosphere.

After the irradiation of the laser light, the melted or sintered metalnanoparticles and the softened or melted principal surface 512 are fusedwith each other. That is, the melted or sintered metal nanoparticlescontacts the softened or melted principal surface 512 to form a bondinginterface including the circuit side entering part and the substrateside entering part described above.

Further, in the circuit part/terminal forming process S204, the terminal530 that contacts with the circuit part 520 is formed. The terminal 530may be formed by irradiating the ink film with the laser light like thecircuit part 520, or may be formed by screen printing without beingirradiated with the laser light.

Then, in the cleaning process S206, the ink on the ink film that is notirradiated with the laser light is washed away and the portion on theink film 110 irradiated with the laser light is dried. In the firingprocess S208, the substrate 510 on which the circuit part 520 and theterminal 530 are formed is fired by using, for example, a blowerfurnace. In the coating part forming process S210, the coating part 540is formed so as to coat the circuit part 520 and the terminal 530. As aresult, the product type circuit board 500 is formed.

Next, in the inspection process S212, the circuit board 500 isinspected. After the inspection, the circuit board 500 is packed andshipped. A series of production processes of the circuit board 500according to the present exemplary embodiment is completed.

In the production method of the circuit board 500 described above, thecircuit part 520 is formed by irradiating the ink film with the laserlight after forming the ink film on the circuit board 500. In such acase, it is possible to inexpensively produce a large amount of thecircuit boards 500 without using patterned ground processing, patternedmask, or the like of the circuit part.

Further, according to the production method of the circuit board 500described above, the circuit part 520 formed on the polyimide substrate510 by heating the ink film with the laser light is reliably bonded tothe surface of the substrate 510. As a result, even when the circuitboard 500 is repeatedly bent, the circuit part 520 is resistant to beingpeeled from the substrate 510, and it is also possible to suppress adecrease in the pulse life.

The variation described with respect to the fuse 1 can be applied to thecircuit board 500 of the present exemplary embodiment.

The present invention is explained with the exemplary embodiments of thepresent invention but the technical scope of the present invention isnot limited to the scope described in the above embodiment. It isapparent for those skilled in the art that it is possible to makevarious changes and modifications to the embodiment. It is apparent fromthe description of the scope of the claims that the forms added withsuch changes and modifications are included in the technical scope ofthe present invention.

What is claimed is:
 1. A fuse comprising: a substrate containing atleast an organic substance; a fuse film that is connected to a principalsurface of the substrate; and both a first entering part formed on abonding interface of the fuse film and a second entering part formed onthe principal surface of the substrate, the first entering part enteringthe inside of the substrate, and the second entering part entering theinside of the fuse film.
 2. The fuse according to claim 1, wherein thefirst entering part is engaged with the inside of the substrate.
 3. Thefuse according to claim 2, wherein the first entering part has a hookshape.
 4. The fuse according to claim 2, wherein a width of a distal endside of the first entering part is larger than a width of a root partside of the first entering part.
 5. The fuse according to claim 1,wherein the substrate is a polyimide substrate.
 6. The fuse according toclaim 1, further comprising connection terminals connected to theprincipal surface of the substrate so as to be connected with the fusefilm at each end of the fuse film in the longitudinal direction, whereinthe thicknesses of the connection terminals are the same as thethickness of the fuse film.